1. Field of the Invention
This invention relates to implantable devices used to defibrillate and pace the heart and more specifically to systems and methods for providing such defibrillation and pacing that transmit energy acoustically from an implanted transmitter to an implanted receiver without the use of conventional lead/electrode systems.
Currently available implantable cardioverter-defibrillator (ICD) devices apply an electrical defibrillation shock to the heart through a lead wire having an electrode, such lead wire and electrode typically being placed within the right ventricle of the heart, and with the shock typically being delivered between this first electrode and the posterior side of the implanted ICD pulse generator housing, which acts as a second electrode. Alternatively, the shock may be delivered between two electrodes on the lead wire, or between the two electrodes and the ICD pulse generator. Current ICD devices also deliver both bradycardia pacing and tachycardia pacing via a bipolar electrode pair located at the distal end of the lead wire in the right ventricle. The lead wires used in pacemakers and ICDs are subject to failure and may cause cardiac perforation, thrombo-occlusion, and infections. Several years after lead implantation, lead failure, due to fracture or insulation break, has been reported to occur in a significant fraction of the patient population.
An ICD-type device currently under development does not require intracardiac lead wires (Cameron Health, Inc., San Clemente, Calif.). Instead of implanting leads in the heart, the device is implanted under the skin of the patient in the left chest area, with one electrode incorporated into the outer body of the implanted device and a second electrode disposed on a subcutaneous lead. These electrodes are situated to deliver the defibrillation energy approximately across the heart, through the chest wall in a fashion similar to a conventional external defibrillator.
Typically, an ICD delivers a shock with a total energy of 3 to 35 Joules, where the leadless subcutaneous device under development is reported to deliver a shock of more than 35 Joules. In either case, due to the location of one or more electrodes (the pulse generator case for a conventional leaded ICD, or the multiple electrodes outside of the chest wall for the subcutaneous device) being located within or close to the musculature of the chest, the delivery of the shock results in skeletal muscle contraction. This can be extremely painful for the patient. While this may not be a concern in the case of ventricular fibrillation as the patient is typically unconscious, ICDs are also programmed to provide shocks to terminate ventricular tachycardia, where the patient is, typically, fully conscious. In fact, the majority of shocks delivered by such ICDs are for the conversion of ventricular tachycardia, before it degenerates into ventricular fibrillation. Furthermore, it would be desirable to utilize ICD-type devices to terminate atrial fibrillation, a condition which is not immediately life threatening, but is associated with increased morbidity and is an uncomfortable condition for most patients. The pain associated with the shock delivery of present ICDs has essentially precluded their use to treat this condition.
Another potentially serious drawback of conventional ICD technology is the latency between the detection of an arrhythmia and the ability to deliver a defibrillation shock. This time-lag is associated with the charging of a large storage capacitor from a current-limited battery, and is typically over 5 seconds with a fully charged battery, and can be 10 to 12 seconds with a device nearing the end of its battery life. Such a long time lag between the detection and defibrillation could be the source of serious, and some times fatal, consequences.
Thus, because of the known problems associated with the use of lead wires, it would be desirable to eliminate the requirement for these lead wire/electrode systems used in present ICDs. Further, it would be desirable to provide shock-based treatment for ventricular and atrial fibrillation, and ventricular tachycardia, with less pain associated with shock delivery, and by providing that treatment internal to the heart without requiring transmission of the shock energy through the musculature of the chest. Additionally, it would be desirable to provide the defibrillation or cardioversion therapy with less latency between the detection of an arrhythmia and the application of the shock.
2. Description of the Background Art
References: U.S. Pat. No. 6,266,567, Ishikawa et al., Implantable Epicardial Electrode, Jul. 24, 2001; U.S. Patent Application Publication US2002/0183791A1, Denker et al., Implantable Defibrillator with Wireless Vascular Stent Electrodes; U.S. Pat. No. 6,647,292, Bardy et al., Unitary Subcutaneous Only Implantable Cardioverter-Defibrillator and Optional Pacer, Nov. 11, 2003; U.S. Pat. No. 6,721,597, Bardy et al., Subcutaneous Only Implantable Cardioverter-Defibrillator and Optional Pacer, Apr. 13, 2004; U.S. Patent Application Publication US2005/0038474A1, Wool, Implantable Automatic Defibrillator with Subcutaneous Electrodes, Feb. 17, 2005; Singer, Igor (editor), Implantable Cardioverter-Defibrillator, Futura Publishing Co., Inc., New York, 1994.